Many games block the SendInput API that XMBC uses to inject keys, because injecting keys can be used to cheat or gain an advantage. That is not the reason for XMBC but as with anything, tools can be used for good and bad depending on the moral compass of the user.

You need to talk to the game developer and persuade them. rather than me, that blocking a tool that many people use with legitimate uses (especially those who are disabled in some way) might be a bad idea.